The present invention relates to devices used to alter the electrical charge distributions of aerosols, and more particularly devices that utilize a corona discharge to generate ions, which then are merged with an aerosol to either charge or neutralize the aerosol.
The study of aerosols involves a variety of applications in which it is desired to adjust the charges on the particles or droplets of the aerosol. There are applications in which it is advantageous to provide a charge distribution in which positive and negatively charged particles counterbalance one another, i.e., an equilibrium charge distribution. In other applications, it is considered more important that a predominant number of the particles carry no charge. In yet further applications, researchers skew the charge distribution toward either the positive or the negative side, and in a more specific application of this type attempt to maximize the number of particles that carry a specific non-zero charge. Corona discharge can be used in all of these applications.
To produce a corona discharge, a non-uniform electrostatic field is established between an electrically conductive needle and a conductive structure proximate the needle, e.g., a plate or a tube surrounding the needle. Given a sufficient field strength, air near the needle experiences a breakdown and becomes conductive. In the conductive corona region, accelerated electrons collide with air molecules to create a dense cloud of free electrons and positive ions. If the needle is biased to a positive voltage relative to the surrounding structure, the electrons return to the needle while the positive ions stream away from the needle toward the adjacent structure. When the discharge needle is disposed within a gas stream, many of the ions do not reach the adjacent structure, but instead become entrained in the gas stream and are transported by the gas stream toward the aerosol. When the discharge needle is negatively biased, the free electrons leave the needle, some of them attaching to molecules of the gas to form negative ions, and are transported toward the aerosol by the gas stream.
In an increasing number of aerosol studies, it is desired to generate aerosols in which the particles are monodisperse, i.e., substantially uniform in size. For these applications, an electrospray nebulizer is preferred, due to its ability to generate small and uniform droplets. In an electrospray nebulizer, an electrically conductive liquid is supplied at a controlled rate to a capillary tube. A voltage differential between the capillary tube and a surrounding conductive wall creates an electrostatic field that induces a surface charge in the liquid emerging from the tube. Electrostatic forces disperse the liquid into a fine spray of charged droplets.
To produce the spray, the droplets are charged near the xe2x80x9cRayleighxe2x80x9d limit, i.e., near the charge at which electrostatic repulsion would overcome the surface tension that otherwise holds the droplet together. As each electrospray droplet evaporates, the charge density on its surface increases, eventually exceeding the Rayleigh limit, causing a disintegration of the droplet into smaller droplets. The droplet fragments in turn continue to evaporate and can experience further disintegration. As a result, the distribution of droplet sizes lacks the uniformity desired for analysis of residue within the droplets.
To counteract this tendency, the droplets are charge neutralized, beginning immediately or shortly after their formation. In one approach, disclosed in U.S. Pat. No. 5,247,842 (Kaufman, et al.), radioactive Polonium is placed inside a chamber through which the electrospray generated droplets travel as they evaporate. The Polonium produces radiation that ionizes air molecules, which in turn encounter the droplets and reduce their charge. This enhances uniformity of the droplets by counteracting their tendency to disintegrate due to electrostatic forces.
This approach yields a reproducible charge distribution by exposing the aerosol particles or droplets to a bipolar plasma of gas ions, both positive and negative, allowing the aerosol elements to reach a steady state of charge distribution. This distribution is useful because it is predictable and produces a large proportion of particles having no charge. This approach has disadvantages, however, in that the use of radioactive materials raises safety and regulatory concerns. The cost of radioactive Polonium is relatively high, and its half-life is relatively short. Further, although the level of ion production can be varied by partially shielding the radioactive material, the level of ionization cannot be precisely controlled.
In view of the above, ion generation through use of a corona discharge has been considered as an alternative method of neutralizing electrospray droplets. The corona discharge can generate unipolar (e.g., only negative) ions, and thus be configured to counteract the charge of the electrospray droplets. Alternatively, if both positive and negative ions are desired, corona discharge devices can have oppositely charged corona discharge needles, or a single corona discharge tip can be rapidly alternated between positively and negatively charged states.
A disadvantage of corona discharge devices is their tendency to generate aerosol particles. The problem is thought to arise from the removal of material from the discharge needle, the creation of highly reactive gaseous species at concentrations sufficient to allow their aggregation into particles, or a combination of these factors. In any event, aerosols generated by the corona discharge needle interfere with attempts to measure the aerosol under study. The tendency especially interferes with the analysis of extremely fine particles, i.e., particles having diameters of about ten nanometers or less.
Particle generation by corona discharge devices interferes with their use in semiconductor manufacturing clean rooms, because the particles can be large enough to contaminate silicone wafers during their processing. In recognition of the problem, U.S. Pat. No. 4,967,608 (Yost) a describes a test chamber for measuring particles larger than three nanometers in diameter emitted from a corona discharge device. U.S. Pat. No. 5,447,763 and U.S. Pat. No. 5,650,203, both issued to Gehlke, recommend selecting certain materials for corona discharge tips, e.g., titanium, aluminum, and other metals that form protective oxide layers. Silicone coated tips of these materials were favored. Platinum and tungsten also were considered, but said to show substantial particle production, and thus found unsatisfactory.
Recently, electrostatic generation of droplets has been considered as a source of aerosols subject to analysis by mass spectrometry, given the capability of generating aerosol droplets that are small (submicron) and monodisperse. In addition, the ability to rapidly and efficiently neutralize the aerosol, preferably to the point where the aerosol consists predominantly of singly charged particles, is a key factor when the aerosol is provided to a mass spectrometer. Although the aforementioned Kaufman patent discloses both the droplet generation and neutralizing beneficial in this regard, a more efficient and more controllable neutralizing of the aerosol would considerably enhance the utility of electrospray-ionization mass spectrometry.
Therefore, it is an object of the present invention to provide an aerosol system in which the charged droplets or charged particles are neutralized more rapidly and in a manner that affords more control over the degree of neutralizing.
Another object is to provide a corona discharge device capable of selectively altering the charge distributions of aerosols formed of extremely small droplets and particles, without generating its own detectable particles and thereby interfering with an analysis of the aerosol under study.
A further object is to provide an electrospray-ionization mass spectrometry system in which the electrostatically generated aerosol is effectively neutralized without requiring the use of radioactive materials.
Yet another object is to provide a corona discharge device particularly well suited for charging and neutralizing aerosols consisting of submicron droplets or particles.
To achieve these and other objects, there is provided a system for generating a charge-adjusted aerosol. The system includes an enclosure defining a mixing chamber, a first orifice for admitting an aerosol into the chamber, and a second orifice for admitting corona discharge ions into the chamber. The system further includes an electrostatic droplet generator having an electrostatic discharge adapted to generate multiple electrically charge droplets of a sample that includes an electrically conductive liquid and a non-volatile material dispersed substantially uniformly throughout the liquid. An ion generator of the system has a corona discharge region electrically biased to generate multiple ions. A fluid passage is adapted for a coupling with a gas source to guide a first gas flow past the electrostatic discharge region. This allows the gas flow to entrain at least a portion of the charged droplets and form an aerosol of the sample, and to carry the entrained droplets into the mixing chamber through the first orifice to direct an aerosol jet into the chamber.
A second fluid passage is adapted for a coupling with a gas source to guide a second gas flow past the corona discharge region. This allows the second gas flow to entrain at least a portion of the ions and carry the entrained ions into the mixing chamber through a second orifice to direct an ion carrying jet into the mixing chamber. The aerosol jet and the ion carrying jet merge inside the mixing chamber in a turbulent flow that promotes the mixing of the charged droplets and the ions, to alter the droplet charges toward a neutralizing of the aerosol. The enclosure further defines an exit orifice permitting the aerosol to exit the mixing chamber after the altering of the droplet charges.
As used in this application, the term xe2x80x9cneutralizingxe2x80x9d refers to a reductionxe2x80x94not the complete removalxe2x80x94of the charges in the particles, droplets or other elements of the aerosol. In this sense, a xe2x80x9cneutralizedxe2x80x9d aerosol can include both charged and neutral (uncharged) particles or droplets. An aerosol with an unbalanced electrical charge distribution can be neutralized in the sense of reducing the predominance of a positive (or negative) charge.
The degree of neutralization varies with the nature of the analytical application. Some applications require neutralization levels sufficient to prevent Coulomb disintegration of charged droplets as they evaporate. In other applications, droplet disintegration may be of no concern, but there may be a need to insure that the number of particles carrying more than a single charge is insignificant. Other applications might require a balanced charge distribution, with or without any limit on the number of charges carried by any given particle.
The preferred ion generator includes an electrically conductive needle providing the corona discharge region. The needle is mounted within an electrically conductive ion generating housing and electrically biased with respect to the housing to provide a corona current, preferably maintained within a range of 10-20 microamperes. The needle, at least along the corona discharge region, is formed of a noble metal, in particular either platinum or a platinum iridium alloy. Other metals of the platinum family may be suitable, although less preferred.
Several factors are believed to contribute to the virtual elimination of measurable particle generation by the corona discharge needle. These include the needle material, the relatively low corona current, and the relatively high velocity gas flow (usually air) past the needle. The rapid air flow tends to cool the discharge needle, which may be a key factor in preventing the particle generation of a platinum needle discussed in the aforementioned Gehlke patents. The airflow also may avoid or reduce the bombardment of the discharge needle tip by corona ions, which otherwise would tend to heat the tip, perhaps sufficiently to evaporate material and thereby generate particles. Furthermore, the active species formed in the corona discharge may be diluted by the rapid airflow before they can aggregate into particles. Finally, the lower corona current contributes to the reduced discharge needle temperature by generating less heat in the needle.
Along with the virtual elimination of corona generated particles, the present system provides for a more efficient and more controllable neutralizing of charged droplets. The turbulence caused by the merger of the aerosol and ion jets effectively mixes the charged droplets and the ions, considerably reducing the time required for a significant number of oppositely charged ions to encounter and reduce the charges of the droplets. The ability to adjust the degree of electrical charge or bias applied to the discharge needle affords a degree of control not available when radioactive ion sources are employed.
An additional advantage of the corona discharge needle in neutralizing electrospray droplets is that it provides a unipolar ion source. If desired, however, the corona discharge can provide a bipolar source of ions, either by providing two oppositely charged corona discharge needles in separate chambers, or by rapidly switching between alternative positive and negative biasing sources coupled to a single discharge needle.
When an aerosol jet and a single ion carrying jet are directed into the mixing chamber, the two jets preferably confront one another and travel in opposite directions towards one another to maximize the mixing potential. Preferably, the jets travel into the chamber at mean velocities of at least 40 meters per second, to insure rapid mixing within a turbulent flow. In an alternative embodiment, a second ion generator provides oppositely charged ions entrained in a third gas flow, resulting in a merger of the aerosol jet with two jets of ions, oppositely charged. In this arrangement, the ion jets are advantageously arranged to confront one another and thus travel in opposite directions while the aerosol jet is perpendicular to the ion jets.
Further in accordance with the invention, there is provided a device for adjusting the electrical charge distribution of an aerosol. An enclosure of the device defines chamber, a first orifice for entry of an aerosol flow, and a second orifice for entry of ions. The device includes an ion generator having a corona discharge region disposed proximate the second orifice and electrically biased to generate multiple ions. A fluid passage is adapted for a coupling with a gas source to guide a gas flow past the corona discharge region. Thus the gas flow entrains some of the first ions and carries the entrained ions into the chamber through the second orifice, to merge with an aerosol flowing into the chamber through the first orifice. When merging with the aerosol, the ions alter the electrical charge distribution of the aerosol. A conductive member is provided proximate the corona discharge region and the second orifice. The member is electrically biased, and has the same electrical polarity as the corona discharge region. The enclosure further has an exit orifice to allow aerosol to exit the enclosure after the electrical charge distribution is altered.
The preferred conductive member is a conductive plate, through which the second orifice is formed. The plate is charged or biased at a level considerably lower than that of the discharge needle, e.g., several hundred volts as compared to the 2,000 volt potential at the needle. Applying a negative charge to the plate, when the corona discharge needle also is negatively charged, has a significant impact. When the plate is negatively charged, the ions are capable of depositing sufficient negative charges on the aerosol particles to produce a larger peak for singly charged negative particles than for singly charged positive particles. The result is an increased fraction of the aerosol particles having a single negative charge, to more than 25 percent, depending on particle size, considerably higher than the fraction obtained by any other charging method. At the same time, a doubly-charged peak on the negative side is avoided, reducing the complexity of the mass spectrum in a manner not possible in systems using either radioactive source ionization, or negative-ion corona sources that lack the biased orifice plate.
Thus, in accordance with the present invention, aerosol analyzing systems can employ an improved corona discharge device that affords a more rapid and more effective altering of the electrical charge distribution of an aerosol, whether to charge or to neutralize the aerosol. The ionizer requires no radioactive ion source, and virtually eliminates the problem of small particle generation found in conventional corona discharge devices. In an electrospray-ionization mass spectrometry system utilizing the ionizer in combination with electrospray generated aerosols, extremely small particles with a charge distribution dominated by neutral and singly charged particles can be provided to the mass spectrometer for analysis.